Method and apparatus for measuring a parameter of a vehicle electrical system

ABSTRACT

An apparatus for measuring electrical parameters for an electrical system measures a first and second parameters of the electrical system between connections to the electrical system. A processor determines a third electrical parameter of the electrical system as a function of the first parameter and the second parameter.

The present application is a Divisional of and claims priority of U.S. patent application Ser. No. 10/656,526, filed Sep. 5, 2003, the content of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to the measurement of electrical parameters of a vehicle electrical system. More specifically, the present invention relates to measuring an electrical parameter of an electrical system of a vehicle through the use of multiple measurements.

Electrical systems, such as those which are used in automotive vehicles, consist of a number of discreet components or systems which are interconnected. Techniques for measuring and utilizing parameters, such as dynamic parameters, of electrical systems are shown and disclosed in U.S. Pat. No. 3,873,911, issued Mar. 25, 1975, to Champlin; U.S. Pat. No. 3,909,708, issued Sep. 30, 1975, to Champlin; U.S. Pat. No. 4,816,768, issued Mar. 28, 1989, to Champlin; U.S. Pat. No. 4,825,170, issued Apr. 25, 1989, to Champlin; U.S. Pat. No. 4,881,038, issued Nov. 14, 1989, to Champlin; U.S. Pat. No. 4,912,416, issued Mar. 27, 1990, to Champlin; U.S. Pat. No. 5,140,269, issued Aug. 18, 1992, to Champlin; U.S. Pat. No. 5,343,380, issued Aug. 30, 1994,; U.S. Pat. No. 5,572,136, issued Nov. 5, 1996; U.S. Pat. No. 5,574,355, issued Nov. 12, 1996; U.S. Pat. No. 5,585,416, issued Dec. 10, 1996; U.S. Pat. No. 5,585,728, issued Dec. 17, 1996; U.S. Pat. No. 5,589,757, issued Dec. 31, 1996; U.S. Pat. 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No. 09/756,638, filed Jan. 8, 2001, entitled METHOD AND APPARATUS FOR DETERMINING BATTERY PROPERTIES FROM COMPLEX IMPEDANCE/ADMITTANCE; U.S. Ser. No. 09/862,783, filed May 21, 2001, entitled METHOD AND APPARATUS FOR TESTING CELLS AND BATTERIES EMBEDDED IN SERIES/PARALLEL SYSTEMS; U.S. Ser. No. 09/880,473, filed Jun. 13, 2001; entitled BATTERY TEST MODULE; U.S. Ser. No. 09/993,468, filed Nov. 14, 2001, entitled KELVIN CONNECTOR FOR A BATTERY POST; U.S. Ser. No. 10/042,451, filed Jan. 8, 2002, entitled BATTERY CHARGE CONTROL DEVICE; U.S. Ser. No. 10/109,734, filed Mar. 28, 2002, entitled APPARATUS AND METHOD FOR COUNTERACTING SELF DISCHARGE IN A STORAGE BATTERY; U.S. Ser. No. 10/112,998, filed Mar. 29, 2002, entitled BATTERY TESTER WITH BATTERY REPLACEMENT OUTPUT; U.S. Ser. No. 10/263,473, filed Oct. 2, 2002, entitled ELECTRONIC BATTERY TESTER WITH RELATIVE TEST OUTPUT; U.S. Ser. No. 10/310,385, filed Dec. 5, 2002, entitled BATTERY TEST MODULE; U.S. Ser. No. 10/462,323, filed Jun. 16, 2003, entitled ELECTRONIC BATTERY TESTER HAVING A USER INTERFACE TO CONFIGURE A PRINTER; U.S. Ser. No. 10/653,342, filed Sep. 2, 2003, entitled ELECTRONIC BATTERY TESTER CONFIGURED TO PREDICT A LOAD TEST RESULT; U.S. Ser. No. 10/656,526, filed Sep. 5, 2003, entitled METHOD AND APPARATUS FOR MEASURING A PARAMETER OF A VEHICLE ELECTRICAL SYSTEM; U.S. Ser. No. 10/441,271, filed May 19, 2003, entitled ELECTRONIC BATTERY TESTER; U.S. Ser. No. 09/653,963, filed Sep. 1, 2000, entitled SYSTEM AND METHOD FOR CONTROLLING POWER GENERATION AND STORAGE; U.S. Ser. No. 10/174,110, filed Jun. 18, 2002, entitled DAYTIME RUNNING LIGHT CONTROL USING AN INTELLIGENT POWER MANAGEMENT SYSTEM; U.S. Ser. No. 10/258,441, filed Apr. 9, 2003, entitled CURRENT MEASURING CIRCUIT SUITED FOR BATTERIES; U.S. Ser. No. 10/681,666, filed Oct. 8, 2003, entitled ELECTRONIC BATTERY TESTER WITH PROBE LIGHT; U.S. Ser. No. 10/748,792, filed Dec. 30, 2003, entitled APPARATUS AND METHOD FOR PREDICTING THE REMAINING DISCHARGE TIME OF A BATTERY; U.S. Ser. No. 10/783,682, filed Feb. 20, 2004, entitled REPLACEABLE CLAMP FOR ELECTRONIC BATTERY TESTER; U.S. Ser. No. 10/791,141, filed Mar. 2, 2004, entitled METHOD AND APPARATUS FOR AUDITING A BATTERY TEST; U.S. Ser. No. 10/864,904, filed Jun. 9, 2004, entitled ALTERNATOR TESTER; U.S. Ser. No. 10/867,385, filed Jun. 14, 2004, entitled ENERGY MANAGEMENT SYSTEM FOR AUTOMOTIVE VEHICLE; U.S. Ser. No. 10/896,834, filed Jul. 22, 2004, entitled ELECTRONIC BATTERY TESTER; U.S. Ser. No. 10/897,801, filed Jul. 23, 2004, entitled SHUNT CONNECTION TO A PCB FOR AN ENERGY MANAGEMENT SYSTEM EMPLOYED IN AN AUTOMOTIVE VEHICLE; U.S. Ser. No. 10/958,821, filed Oct. 5, 2004, entitled IN-VEHICLE BATTERY MONITOR; U.S. Ser. No. 10/958,812, filed Oct. 5, 2004, entitled SCAN TOOL FOR ELECTRONIC BATTERY TESTER; U.S. Ser. No. 11/008,456, filed Dec. 9, 2004, entitled APPARATUS AND METHOD FOR PREDICTING BATTERY CAPACITY AND FITNESS FOR SERVICE FROM A BATTERY DYNAMIC PARAMETER AND A RECOVERY VOLTAGE DIFFERENTIAL, U.S. Ser. No. 60/587,232, filed Dec. 14, 2004, entitled CELLTRON ULTRA, U.S. Ser. No. 11/018,785, filed Dec. 21, 2004, entitled WIRELESS BATTERY MONITOR; U.S. Ser. No. 60/653,537, filed Feb. 16, 2005, entitled CUSTOMER MANAGED WARRANTY CODE; U.S. Ser. No. 11/063,247, filed Feb. 22, 2005, entitled ELECTRONIC BATTERY TESTER OR CHARGER WITH DATABUS CONNECTION; U.S. Ser. No. 60/665,070, filed Mar. 24, 2005, entitled OHMMETER PROTECTION CIRCUIT; U.S. Ser. No. 11/141,234, filed May 31, 2005, entitled BATTERY TESTER CAPABLE OF IDENTIFYING FAULTY BATTERY POST ADAPTERS; U.S. Ser. No. 11/143,828, filed Jun. 2, 2005, entitled BATTERY TEST MODULE; U.S. Ser. No. 11/146,608, filed Jun. 7, 2005, entitled SCAN TOOL FOR ELECTRONIC BATTERY TESTER; U.S. Ser. No. 60,694,199, filed Jun. 27, 2005, entitled GEL BATTERY CONDUCTANCE COMPENSATION; U.S. Ser. No. 11/178,550, filed Jul. 11, 2005, entitled WIRELESS BATTERY TESTER/CHARGER; U.S. Ser. No. 60/705,389, filed Aug. 4, 2005, entitled PORTABLE TOOL THEFT PREVENTION SYSTEM, U.S. Ser. No. 11/207,419, filed Aug. 19, 2005, entitled SYSTEM FOR AUTOMATICALLY GATHERING BATTERY INFORMATION FOR USE DURING BATTERY TESTER/CHARGING, U.S. Ser. No. 60/712,322, filed Aug. 29, 2005, entitled AUTOMOTIVE VEHICLE ELECTRICAL SYSTEM DIAGNOSTIC DEVICE, U.S. Ser. No. 60/713,169, filed Aug. 31, 2005, entitled LOAD TESTER SIMULATION WITH DISCHARGE COMPENSATION, U.S. Ser. No. 60/731,881, filed Oct. 31, 2005, entitled PLUG-IN FEATURES FOR BATTERY TESTERS; U.S. Ser. No. 60/731,887, filed Oct. 31, 2005, entitled AUTOMOTIVE VEHICLE ELECTRICAL SYSTEM DIAGNOSTIC DEVICE; U.S. Ser. No. 11/304,004, filed Dec. 14, 2005, entitled BATTERY TESTER THAT CALCULATES ITS OWN REFERENCE VALUES; U.S. Ser. No. 60/751,853, filed Dec. 20, 2005, entitled BATTERY MONITORING SYSTEM; U.S. Ser. No. 11/304,004, filed Dec. 14, 2005, entitled BATTERY TESTER WITH CALCULATES ITS OWN REFERENCE VALUES; U.S. Ser. No. 60/751,853, filed Dec. 20, 2005, entitled BATTERY MONITORING SYSTEM; U.S. Ser. No. 11/352,945, filed Feb. 13, 2006, entitled BATTERY TESTERS WITH SECONDARY FUNCTIONALITY; U.S. Ser. No. 11/356,299, filed Feb. 16, 2006, entitled CENTRALLY MONITORED SALES OF STORAGE BATTERIES; U.S. Ser. No. 11/356,436, field Feb. 16, 2006, entitled ELECTRONIC BATTERY TESTER WITH RELATIVE TEST OUTPUT; U.S. Ser. No. 11/356,443, filed Feb. 16, 2006, entitled ELECTRONIC BATTERY TESTER WITH NETWORK COMMUNICATION; U.S. Ser. No. 11/410,263, filed Apr. 24, 2006, entitled QUERY BASED ELECTRONIC BATTERY TESTER; U.S. Ser. No. 11/498,703, filed Aug. 3, 2006, entitled THEFT PREVENTION DEVICE FOR AUTOMOTIVE VEHICLE SERVICE CENTERS; U.S. Ser. No. 11/507,157, filed Aug. 21, 2006, entitled APPARATUS AND METHOD FOR SIMULATING A BATTERY TESTER WITH A FIXED RESISTANCE LOAD; U.S. Ser. No. 11/511,872, filed Aug. 29, 2006, entitled AUTOMOTIVE VEHICLE ELECTRICAL SYSTEM DIAGNOSTIC DEVICE; U.S. Ser. No. 11/519,481, filed Sep. 12, 2006, entitled BROAD-BAND LOW-CONDUCTANCE CABLES FOR MAKING KELVIN CONNECTIONS TO ELECTROCHEMICAL CELLS AND BATTERIES; U.S. Ser. No. 60/847,064, filed Sep. 25, 2006, entitled STATIONARY BATTERY MONITORING ALGORITHMS; which are incorporated herein in their entirety.

There is an ongoing need to measure parameters of electrical systems of vehicles and heavy equipment. Such measurements can be used to diagnose operation, failure or impending failure of components or subsystems of electrical systems. For example, in electrical systems used in vehicles, measurement of electrical parameters of such systems can be used to diagnose operation of system or indicate that maintenance is required before ultimate failure.

One particular measurement is the resistance of cabling used in large equipment such as heavy trucks. For example, one such cable or set of cables connects the battery of vehicle to the starter motor. The starter motor has a relatively large current draw and even a relatively small cable resistance can have a significant impact on operation of the starter motor.

Because the cable resistance is relatively small it typically cannot be measured using a standard ohm meter or other techniques which are normally used to measure resistance. One technique which has been used to measure the cable resistance is to run a very large current through the cable and measure the voltage drop. However, this is cumbersome and requires components capable of handling the large current.

SUMMARY OF THE INVENTION

An apparatus for measuring electrical parameters for an electrical system includes measurement circuitry which is configured to measure a first parameter of the electrical system between a first connection to the electrical system and a second connection to the electrical system. The measurement circuitry is further configured to measure a second parameter of the electrical system between a third connection to the electrical system and the second connection to the electrical system. A processor determines a third electrical parameter of the electrical system as a function of the first parameter and the second parameter. A method can also be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of an electrical system of a vehicle.

FIG. 2 is a diagram showing test equipment for determining the resistance of cables of the electrical system shown in FIG. 1.

FIG. 3 shows another example embodiment of test equipment for determining cable resistance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a diagram of an electrical system 10 of large equipment 12 such as a heavy truck. Electrical system 10 includes a battery 20, a high current load 22 and cables 24 and 26. Cables 24 and 26 have resistances R₁ and R₂, respectively and connect load 22 to battery 20. FIG. 1 also shows connection points C, D and C′, D′. Connections C and D are cross load 22 and connections C′ and D′ are cross battery 20.

As discussed in the Background section, the resistances R₁ and R₂ of cables 24 and 26 can have a significant impact on the amount of power which can be delivered to load 22. Even if the resistance values are relatively small, because a relatively large current passes through cables 24 and 26, the resultant voltage drop can significantly reduce the voltage at points C and D and therefore the amount of power (or voltage) which can be delivered to load 22. In industrial equipment, it is often desirable to measure the resistance R₁ and R₂ of cables 24 and 26, respectively, in order to identify a cable with a resistance which is too high. One technique which has been used to measure the resistance of the cables is to pass a large current through the cable and measure the resulting voltage drop across the cable. However, this is a cumbersome test and requires electrical test equipment which is capable of handling the large current draw. The present invention provides an apparatus and technique for measuring the resistance of a cable in a configurations similar to that shown in FIG. 1.

FIG. 2 is a simplified block diagram of one example embodiment of electrical test equipment 50 for measuring electrical parameters of the electrical system 10 shown in FIG. 1. Test equipment 50 includes measurement circuitry 52, microprocessor 54, memory 56 and output 58. Measurement circuitry 52 is configured to couple to electrical system 10 of FIG. 1 through electrical connections 60 and 62. Measurements obtained by measurement circuitry 52 are used by microprocessor 54 in accordance with program instructions contained in memory 56. Based upon the measurements, an output is provided through output 58, for example, to a user or to other equipment. Connectors 60 and 62 are configured to couple to points C, D and C′, D′ in order to measure parameters of system 10. Any number of connectors may be used and the invention is not limited to the two illustrated in FIG. 2.

In one aspect of the present invention, test equipment 50 measures a parameter P(C,D′) between points C and D′ and a parameter P(C′,D′) between points C′ and D′. These measurements are used to determine the resistance of R₁ in accordance with the formula: R ₁ =F[P(C,D′), P(C′,D′)]  EQ. 1 Further, a third measurement can be taken to obtain a parameter P(C′,D) between points C′ and D in FIG. 1. With this additional parameter, the resistance of R₂ can be determined as: R ₂ =F[P(C′,D), P(C′,D′)}  EQ. 2

Microprocessor 54 can determine the actual values of R₁ and R₂, or can make some other determination related to R₁ and R₂, for example a pass/fail determination, a relative determination, a gradient based determination, etc. Microprocessor 54 provides an output through output 58 based upon the determination related to R₁ and R₂. The output can be a visual output, audible output, or the like, to an operator. In another example, the output is suitable for receipt by other circuitry.

FIG. 3 is a simplified diagram showing another example embodiment of circuitry in accordance with the present invention. In FIG. 3, test equipment 100 includes a microprocessor 54, memory 56 and output 58, similar to the configuration discussed with respect to FIG. 2. Additionally, measurement circuitry 102 is provided for coupling to the C,D and C′,D′ connections shown in FIG. 1. More specifically, Kelvin connections 104 and 106 are provided and are identified as A, B, C and D with connections 104B, 106A, 104A and 106B, respectively. Kelvin connection 104 is configured to couple to location C shown in FIG. 1. Kelvin connection 106 is configured to couple to location D shown in FIG. 1. An additional pair of connections 108 and 110 are configured to couple to locations C′ and D′ shown in FIG. 1. A forcing function 120 couples to connections 104B and 106A (A and B) and is configured to apply a time varying signal therebetween. The signal can be any type of time varying signal including a periodic signal and may have any type of waveform at a desired frequency or multiple frequencies. Further, in some embodiments, measurements are taken using different forcing functions at differing frequencies or waveforms. The forcing function can be an active signal which is injected through the A/B connection, or can be a passive signal in which a signal is drawn from points A/B through selective application of a resistance, etc.

An amplifier 122 couples to connections 104A and 106B (C and D) and provides an output to an analog to digital converter 124. Connections 108 and 110 (C′ and D′) couple to an amplifier 126 which provides an output to analog to digital converter 124. Note that this configuration is for explanation only and other configurations can be implemented in accordance with the present invention including different amplifier configurations, different analog to digital converter configurations, etc. Further, the forcing function 120 can be an active forcing function in which a signal is actively applied or can be a passive forcing function in which a signal is applied passively through a resistance or the like which is selectively applied to draw current from battery 20 shown in FIG. 1. The circuitry can be implemented in analog or digital circuitry, or their combination. Circuitry in accordance with techniques set forth in the Background section can be implemented, or other measurement techniques can be used.

Using the configuration set forth in FIG. 3, Kelvin connections 104 and 106 can be applied to points C and D identified in FIG. 1. Additional connections 108 and 110 can be applied to points C′ and D′ shown in FIG. 1. Using this configuration, the parameters measured in accordance with FIGS. 1 and 2 can be dynamic parameters which are functions of the applied forcing function 120. In another example embodiment, a single pair of Kelvin connections is used in which the connections are moved between various positions C, D, C′ and D′ shown in FIG. 1 and the resistance R₁ and R₂ of the cables 24 and 26 are determined.

Using the circuitry set forth in FIG. 3, conductance values between the various connections shown in FIG. 1 can be obtained. Using these conductance values, the resistances R₁ and R₂ can be determined using the following equations: R ₁=(K ₁ /G _(CD′))−(K ₂ /G _(C′D′))   EQ. 3 R ₂=(K ₃ /G _(C′D))−(K ₄ /G _(C′D′))   EQ. 4 Where G_(CD′) is the conductance measured between points C and D′, G_(C′D′) is the conductance measured between points C′ and D′ and G_(C′D) is the conductance measured between points C′ and D. The values K₁, K₂, K₃ and K₄ are constants and can be, in some examples, the same value, for example unity. The conductance values can be either direct conductance values or can be conductance values converted to a cold cranking amps (CCA) scale. When CCA values are measured, he values of R₁ and R₂ can be determined using the formula: R ₁=(3.125/CCA _(—) CD′)−(3.125/CCA _(—) C′D′)   EQ. 5 R ₂=(3.125/CCA _(—) C′D)−(3.125/CCA _(—) C′D′)   EQ. 6 The value of 3.125 can be adjusted based upon the particular CCA scale employed.

The load 22 can be any type of load including loads which draw high current levels, for example, a starter motor, a magnetic switch, a ground connection, wiring harness, a terminal which may be susceptible to corrosion, a connection through a bolt which may have inappropriate torque or otherwise provide a poor connection, trailer wiring, etc. In one example output, a particular voltage drop is provided for a particular current draw through the cabling. For example, the output can comprise an indication that there is a 0.5 volt drop through the cable under a 500 amp current. Such a parameter can also be used, for example, in a pass/fail test, i.e., if the voltage drop is more than a particular threshold at a given current level, a failure indication can be provided as an output. In one embodiment, the measured parameters comprise dynamic conductance. However, any dynamic parameter can be used in accordance with the present invention including dynamic resistance, reactance, impedance, conductance, susceptance, and/or admittance, including any combination of these parameters.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. The measurements can be taken using multiple connections to the electrical system or by moving a single pair of connections to various positions on the electrical system. An output can be provided to instruct the operator where to place the connections. 

1. An apparatus for measuring electrical parameters of an electrical system comprising: measurement circuitry configured to: measure a first parameter of the electrical system between a first connection to the electrical system and a second connection to the electrical system; measure a second parameter of the electrical system between a third connection to the electrical system and the second connection to the electrical system; and a processor configured to determine a third electrical parameter of the electrical system as a function of the first parameter and the second parameter.
 2. The apparatus of claim 1: wherein the measurement circuitry is further configured to: measure a fourth dynamic parameter between the third electrical connection to the electrical system and a fourth connection to the electrical system; measure a fifth parameter of the electrical system between the third electrical connection and the second electrical connection; and the processor is further configured to determine a sixth parameter of the electrical system as a function of the fourth and the fifth parameters.
 3. The apparatus of claim 1 wherein the first and second parameters comprise dynamic parameters.
 4. The apparatus of claim 1 wherein the first and second parameters are measured in response to a forcing function.
 5. The apparatus of claim 4 wherein the forcing function comprises an active forcing function.
 6. The apparatus of claim 4 wherein the forcing function comprises a passive forcing function.
 7. The apparatus of claim 1 including Kelvin connectors configured to couple to the electrical system.
 8. The apparatus of claim 1 wherein the third electrical parameter comprises electrical resistance of a cable of the electrical system.
 9. The apparatus of claim 1 wherein the electrical system comprises an electrical system of a vehicle.
 10. The apparatus of claim 1 wherein the third parameter comprises electrical resistance and is determined in accordance with the equation: R ₁ =F[P(C,D′), P(C′,D′)]Where C, C′ and D′ are points on the electrical system.
 11. The apparatus of claim 10 including a forcing function applied between the C point on the electrical system and a D point on the electrical system.
 12. The apparatus of claim 1 wherein the first and second parameters are indicative of a cold cranking amps (CCA) measurement.
 13. The apparatus of claim 1 including an output configured to provide an output related to the third electrical parameter.
 14. The apparatus of claim 13 wherein the output comprises an output to an operator.
 15. The apparatus of claim 13 wherein the output comprises an output to electrical circuitry.
 16. The apparatus of claim 13 wherein the output comprises a pass/fail output.
 17. The apparatus of claim 13 wherein the output is indicative of a voltage drop for a particular current through the electrical system.
 18. A method for measuring electrical parameters of an electrical system comprising: measuring a first parameter of the electrical system between a first connection to the electrical system and a second connection to the electrical system; measuring a second parameter of the electrical system between a third connection to the electrical system and the second connection to the electrical system; and determining a third parameter of the electrical system as a function of the first parameter and the second parameter.
 19. The method of claim 18 including: measuring a fourth dynamic parameter between the third electrical connection to the electrical system and a fourth connection to the electrical system; measuring a fifth parameter of the electrical system between the third electrical connection and the second electrical connection; and determining a sixth parameter of the electrical system as a function of the fourth and the fifth parameters.
 20. The method of claim 18 wherein the first and second parameters comprise dynamic parameters.
 21. The method of claim 18 including applying a forcing function and wherein the first and second parameters are measured in response to the forcing function.
 22. The method of claim 21 wherein the forcing function comprises an active forcing function.
 23. The method of claim 21 wherein the forcing function comprises a passive forcing function.
 24. The method of claim 18 including applying Kelvin connectors configured to couple to the electrical system.
 25. The method of claim 18 wherein the third electrical parameter comprises electrical resistance of a cable of the electrical system.
 26. The method of claim 18 wherein the electrical system comprises an electrical system of a vehicle.
 27. The method of claim 18 wherein the third parameter comprises electrical resistance and is determined in accordance with the equation: R ₁ =F[P(C,D′), P(C′,D′)]Where C, C′ and D′ are points on the electrical system.
 28. The method of claim 27 including applying a forcing function between the C point on the electrical system and a D point on the electrical system.
 29. The method of claim 18 wherein the first and second parameters are indicative of a cold cranking amps (CCA) measurement.
 30. The method of claim 18 including providing an output related to the third electrical parameter.
 31. The method of claim 30 wherein the output is provided to an operator.
 32. The method of claim 30 wherein the output is provided to electrical circuitry.
 33. The method of claim 30 wherein the output comprises a pass/fail output.
 34. The method of claim 30 wherein the output is indicative of a voltage drop for a particular current through the electrical system.
 35. An apparatus for measuring electrical parameters of an electrical system comprising: measurement means for: measuring a first parameter of the electrical system between a first connection to the electrical system and a second connection to the electrical system; measuring a second parameter of the electrical system between a third connection to the electrical system and the second connection to the electrical system; and processor means for determining a third electrical parameter of the electrical system as a function of the first parameter and the second parameter. 